Modulator for electromagnetic signals transmitted by a contactless transmission/reception system

ABSTRACT

A data transmission device using electromagnetic wave radiation in a contactless transceiver system toward a contactless object, the data bits transmitted corresponding to the half cycle of an initial time interval during which electromagnetic waves are emitted at a predetermined carrier frequency and to a second splitting time interval during which there is a split in transmission of the electromagnetic waves at the predetermined carrier frequency. During said second splitting time interval, generation means ( 40, 42, 44, 46, 48 ) generate electromagnetic waves having a frequency which is double the predetermined carrier frequency resulting in an attenuation of the radiated field by the antenna greater than a predetermined value such as 30 dB.

This application is a U.S. National Stage of International ApplicationPCT/FR02/03130, filed Sep. 13, 2002 and published on Mar. 27, 2003 inthe French Language.

TECHNICAL FIELD

This invention concerns electromagnetic signal transmission antennaslocated in contactless transceiver system readers and designed to emitelectromagnetic signals toward portable contactless objects, andspecifically concerns a modulator for electromagnetic signals emitted bya contactless transceiver system.

BACKGROUND ART

The exchange of information between a contactless object and acontactless transceiver system is generally accomplished by remoteelectromagnetic coupling between the first antenna located in thecontactless object and the second antenna located in the contactlesstransceiver system. Furthermore, the object is equipped with anelectronic module featuring the first antenna connected to an electronicchip which contains, among other elements, a radio-frequency (RF) part,a microprocessor and/or a memory in which the information to be providedto the contactless transceiver system and the logic functions requiredto compile the information to be transmitted and to process theinformation received.

The contactless object, which may be a ticket or credit card formatcard, is a system which is being increasingly used in various sectors.For example, in the transportation sector, the disposable contactlessticket and the contactless smart card were developed as a means ofpayment for both occasional and regular users. The same holds true forthe electronic wallet. Many companies have also developed identificationmeans for their personnel using contactless smart cards.

At present, data transmissions between the contactless transceiversystem, commonly referred to as the reader, and contactless smart cardsare subject to ISO standards. Among the most widely spread, the standardISO 14443 concerns data transmission via radio between a smart card anda reader and vice versa. This standard covers two transmission protocolsknown as type “A” transmission protocol and type “B” transmissionprotocol. These two data contactless data transmission protocols, A andB, differ in terms of the type of modulation used for Radio Frequency(RF) communication between the reader and the card on the one hand, andthe card and the reader on the other hand. Only the signals transmittedfrom the reader to the card will be dealt with here.

In the direction of data transmission from the reader to the card,protocol B provides amplitude modulation corresponding to a modulationrate of 10 percent of the signal emitted or of the electromagneticcarrier wave by the data transmitted, while in protocol A, theelectromagnetic carrier wave is modulated at 100 percent of itsamplitude by the data transmitted. In both cases, the amplitude of theperiodic electromagnetic carrier wave emitted is maximum by defaultduring the first time interval t1 then, during the second time interval,for the first case, it is equal to approximately 82 percent of themaximum amplitude during the modulation while in the second case, it isequal to 0 percent of the maximum amplitude during the modulation time.

At present, an increasing number of standards require that contactlessreaders to be compatible with both transmission protocol types, A and B.The standardized electromagnetic carrier wave frequency is common toboth protocols and is generally equal to 13.56 MHz.

One of the major performance criteria for a reader is the range of theelectromagnetic radiated field which must be as large as possible. Thus,manufacturers attempt to develop the range of their transmission systemby means other than increasing the voltage source. However, the increasein range must not present the risk of saturating or destroying the cardwhen placed near the reader.

One of the factors for satisfying this performance criterion is the useof antennas which have a high overvoltage ratio. At the resonancefrequency, the rms voltage at the terminals of the inductance, a sourceof electromagnetic carrier waves, is substantially equal to Q times thevoltage at the terminals of the circuit; Q being the overvoltage factor.In this manner, the more the antenna presents a high overvoltage ratio,the greater is the range of its radiated field.

In order to obtain modulation of the signal emitted at 100 percent ofits amplitude, the method commonly used at present consists in switchingthe voltage source off at the terminals of the circuit for the timecorresponding to the split of the field according to protocol A, inorder to stop the electromagnetic carrier wave from being transmitted.

In practice, switching off the generator drops the voltage to zeroalthough significantly increases the impedance of the antenna's drivingcircuit. This results in the antenna continuing to emit owing to theloads accumulated in the circuit, which results in a dampened andoscillating emitted electromagnetic carrier wave for a time greater thanthe splitting time. As a result, during the signal split, the amplitudeof the field radiated toward the card is not zero and the field emittedby the antenna thus does not correspond to a modulation of 100 percentof the amplitude of the electromagnetic carrier wave; this takes placefor a time less than the theoretical split time, the amplitude beingless than or equal to 5 percent of the maximum amplitude of the radiatedsignal.

Consequently, the use of an antenna with a high overvoltage ratio iscompatible with a B type reader. For such a reader, the electromagneticcarrier wave is always active as it is modulated at only 10 percent ofits amplitude while it is poorly compatible with an A type reader.Furthermore, the small dampening effect obtained with an antenna havinga high overvoltage ratio only slightly influences the shape of a wavemodulated at 10 percent of its amplitude.

In order to use an antenna which is compatible for both reader types, afirst solution consists in using an antenna having a reduced overvoltageratio and with damping compatible with the requirements of standard A,although at the expense of the performance characteristics understandard B.

A second solution consists in using a linear amplifier with bipolartransistors, which enables the signal emitted by the antenna to bedampened so that the radiated field is zero during the splitting time.This solution allows the expected damping to be obtained at the expenseof yield. The circuit has constant output impedance, although requireshigh polarization currents. Implementation of the circuit is morecomplex.

A third solution consists in using field effect transistors forswitching, in order to reduce losses and to not penalize the range ofthe B type reader. Using such transistors switches the circuit betweenthe “open” and “closed” positions and allows a maximum radiated field tobe obtained by increasing the impedance presented when the circuit isopen sustainably but at the expense of the wave form in the case of thetype A protocol.

DISCLOSURE OF THE INVENTION

This is why a first object of the invention is to provide a deviceenabling the transmission of an electromagnetic carrier wave from theantenna of the reader toward the contactless smart card under maximumperformance conditions during the emission of type A protocol signals.

A second object of the invention is to provide a device enabling thetransmission of an electromagnetic carrier wave from the antenna of thereader toward the contactless smart card under maximum performanceconditions during the emission of type A and type B protocol signals.

The purpose of the invention thus concerns a data transmission deviceusing electromagnetic wave radiation in a contactless transceiver systemtoward a contactless object, the data bits transmitted corresponding tothe alternation of a first time interval during which electromagneticwaves are emitted at a predetermined carrier frequency and a secondsplitting time interval during which there is a split in thetransmission of the electromagnetic waves at the predetermined carrierfrequency. The device includes generation means for generating, duringthe second splitting time interval, electromagnetic waves with afrequency greater than the fundamental frequency of the split resultingin an attenuation of the radiated field by the antenna greater than apredetermined value.

BRIEF DESCRIPTION OF THE FIGURES

The purposes, objects and characteristics of the invention will becomemore apparent from the following description when taken in conjunctionwith the accompanying drawings in which:

FIG. 1 represents the envelope of the field emitted according toprotocol A, obtained by splitting the frequency of the carrier wave,

FIG. 2 represents the envelope of the field emitted according to theinvention,

FIG. 3 represents the amplitude curve of the field radiated by theantenna in relation to the frequency of the signal received,

FIG. 4 is a block diagram of the functions of the device according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

The data are transmitted from the reader to the smart card via RFsignals according to the two data transmission protocol types A and Bsubject to current standards. Type A protocol modulates theelectromagnetic wave emitted by the reader at 100 percent of itsamplitude. The amplitude of the periodic carrier frequency wave ismaximum and constant during an initial time interval then zero during asecond time interval. Type B protocol modulates the electromagnetic waveemitted by the reader at 10 percent of its amplitude. The amplitude ofthe periodic carrier frequency wave is maximum and constant during aninitial time interval then equal to 10 percent of its maximum amplitudeduring a second time interval. As a de facto standard, the frequency ofthe electromagnetic carrier wave is currently equal to 13.56 MHz andthis is why this value is chosen as the reference value in thisdocument. However, the frequency of the electromagnetic carrier wave maybe different without deviating from the scope of the invention.

According to type A data transmission protocol, the electromagneticcarrier wave is split for a time interval t by splitting of the 13.56MHz transmitting frequency.

The envelope of the field thus radiated by the reader's antenna isrepresented in FIG. 1. While the frequency of the carrier frequency isequal to 13.56 MHz, the radiated field 10 is maximum. When the frequencyis split, the radiated field decreases progressively according to thecurve 12 until it reaches a minimum value other than zero at the end ofthe time interval t. At the end of the time interval t, the frequency ofthe carrier wave is again equal to 13.56 MHz and the amplitude of theradiated field increases according to the curve 14 until it reaches itsmaximum 10. In this manner, the modulation obtained from the amplitudeof the carrier wave is not satisfactory when it is a modulation at 100percent of the maximum amplitude. Such modulation is obtained when aminimum value of the amplitude of the radiated field is obtained (thestandard currently requires less than 5 percent of the maximumamplitude) during a certain time in the time interval t. Now, when thecarrier wave frequency represented by curve 12 is split, the reductionof the radiated field is not significant enough to obtain a satisfactorywave form in terms of modulation at 100 percent of the maximumamplitude.

FIG. 2 represents the envelope of the field radiated by the reader'santenna by the device according to the invention. While the frequency ofthe carrier frequency is equal to 13.56 MHz, the radiated field 20 ismaximum. In order to obtain a split of the amplitude of the carrier waveduring time t, the frequency of the signal sent toward the antenna isthus double the resonance frequency during time t. The antenna thusreceives a signal having a frequency of 27.12 MHz and no field isradiated by the antenna as this frequency is defined as being outsidethe antenna's bandwidth. In reality, the radiated field obtained duringthe frequency change decreases rapidly along a very damped curve 22until it reaches a negligible value during a significant time t′. Whenthe frequency of the electromagnetic carrier wave is 13.56 MHz, thefield 24 radiated by the antenna increases until it reaches its maximumvalue 20. The wave-shape obtained is thus satisfactory in terms ofmodulation at 100 percent of the maximum amplitude. This result may beobtained with a frequency of the signal emitted outside the antenna'sbandwidth and greater than the fundamental frequency of the split.

The field radiated by the antenna is illustrated in FIG. 3 according tothe frequency of the signal received by the antenna. This field ismaximum at the resonance frequency f_(R). For a frequency less than orequal to the frequency f₁, the amplitude of the radiated field is lessthan 5 percent of the amplitude of the radiated field at the resonancefrequency f_(R). In the same manner, f₂ represents the frequency abovewhich the amplitude of the radiated field is less than 5 percent of theamplitude of the field radiated at the resonance frequency. At thefrequency generated during the split satisfactory to obtain a variablefield split according to protocol A, the amplitude of the field radiatedby the antenna must be less than 5 percent of the maximum amplitude ofthe radiated field. The frequency generated during the cut-off must thusbe such that it results in an attenuation of the radiated field equal toa predetermined value, this value preferably being equal to 30 dB. Thefrequency generated during the split is either less than f₁ and otherthan zero, or greater than f₂. In a preferred embodiment, it is greaterthan or equal to 2 times f_(R). According to the preferred embodiment ofthe invention, the frequency generated during the split is equal to twotimes the resonance frequency, that is 27.120 MHz.

The electronic device shown in block diagram format in FIG. 4 representsa preferred embodiment of the invention in which the data signal 42controls a two-position switch 48. The first position corresponds to asignal whose frequency is equal to 27.120 MHz generated during the splitprovided by a clock 44. The second position corresponds to a signalwhose frequency is equal to the standard frequency of 13.56 MHzgenerated by a frequency divider 46. The means above are included inprogrammable logic 40, the output signal 50 of which is applied to thepower switching stage by two inputs, input 52 corresponding to theoutput signal of the programmable logic 40 while the other input 54 isthis same output signal inverted by a change-over switch 53. The powerswitching stage 56 includes two field effect transistors connected toboth inputs 52 and 54 respectively and intended to switch to thefrequency received by one of the two inputs, one of the transistorsbeing “open” while the other transistor is “closed”. Both output signalsfrom the power switching stage pass by two resistors 58 and 60 beforebeing filtered by a low-pass filter 62 designed to allow onlyfundamental frequency signals through. On the output of filter 62, thetwo signals are emitted by the antenna 64 toward the contactless card.

1. A transmission device using electromagnetic wave radiation in a contactless transceiver system toward a contactless object, data bits transmitted corresponding to alternation of a first time interval during which electromagnetic waves are emitted at a predetermined carrier frequency and a second splitting time interval during which there is a split in transmission of the electromagnetic waves at said predetermined carrier frequency, further comprising generation means for generating, during said second splitting time interval, electromagnetic waves with a frequency greater than the fundamental frequency of the split resulting in an attenuation of the radiated field by the antenna greater than a predetermined value.
 2. The data transmission device of claim 1, wherein said attenuation of the radiated field at said frequency generated during the split is less than or equal to 30 dB.
 3. The data transmission device of claim 2, wherein said frequency generated during the split is outside a frequency range f1–f2 corresponding to an amplitude of the field radiated by the antenna greater than or equal to 5 percent of the amplitude of the field radiated at said predetermined carrier frequency.
 4. The data transmission device of claim 3, wherein said frequency generated during the split is equal to double of said predetermined carrier frequency.
 5. The data transmission device of claim 4, further comprising programmable logic supplying a first signal at said predetermined carrier frequency for an initial time interval and a second signal having a frequency which is double that of said predetermined carrier frequency during a second time interval, the switching between said first signal and said second signal being controlled by the data signal.
 6. The data transmission device of claim 5, wherein said predetermined carrier frequency is equal to 13.56 MHz.
 7. The data transmission device of claim 6, wherein said programmable logic includes a clock operating at 27.12 MHz and a frequency divider supplying said predetermined carrier frequency (13.56 MHz), the double frequency (27.12 MHz) being provided by said clock.
 8. The data transmission device of claim 7, further comprising a power switching stage featuring two field effect transistors controlled by the outputs of said programmable logic such that one of said transistors is “open” while the other transistor is “closed”.
 9. The data transmission device of claim 1, wherein the data transmitted are compliant with the data transmission standard ISO 14443 according to type A protocol in which the data bits are generated by splits of the electromagnetic field emitted. 